Cryptic crossword clue
Prime ploy devised to detain King in unseemly fashion 10

The most common and most likely answer to this crossword clue is the 10 letter word IMPROPERLY

I've seen this crossword clue in The Daily Telegraph
I
M
P
R
O
P
E
R
L
Y

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The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.
This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.

What does IMPROPERLY mean?

IMPROPERLY adverb
  1. In an improper manner; not properly

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